Monday, May 14, 2018

“Fusion is the ultimate goal of energy research. It is clean, abundant, and safe,” says Dr. Luke Ceurvorst, a researcher at the University of Bordeaux in France. Recently, Ceurvorst and a team of collaborators from around the world reported new research results in the American Physical Society’s journal Physical Review E that will help scientists working to achieve nuclear fusion using a technique called fast ignition.

This experiment took place at the OMEGA Extended Performance (EP) Laser Facility at the University of Rochester, shown here. Image Credit: P.M. Nilson, University of Rochester.

Inspired by nature, the idea behind fusion power is simple but powerful. The core of the sun is a natural fusion reactor. It’s very dense and incredibly hot—27 million degrees Fahrenheit. Because of these extreme conditions, the matter at the sun’s core is a plasma, a mixture of positively charged ions and negatively charged electrons. Within the plasma, the particles zip around randomly at super high speeds. Many of these particles are protons, the nuclei of hydrogen atoms.

When protons in the plasma slam into one another with enough force to overcome their natural repulsion, they can fuse together—first into the nucleus of a "heavy hydrogen" atom called deuterium, then into the nucleus of a helium atom. A small amount of mass is lost in this process, which—as demonstrated by Einstein’s E=mc2—becomes a lot of energy. Some of this energy helps sustain fusion reactions by keeping the sun's core hot. The rest of the energy eventually leaves the sun, giving us the light and heat that make life possible on Earth.

If we could create fusion reactors on Earth that mimic this process of fusing two light nuclei into a heavier nucleus, we could have a potentially limitless supply of clean energy: pound for pound, fusing hydrogen releases ten million times more energy than burning gasoline. Gone would be the days of fighting over fossil fuels and polluting the Earth with their emissions. There would be no danger of nuclear meltdowns, and weapons research facilities couldn’t hide under the guise of energy research facilities. It would be a turning point in human history.

For these reasons, fusion power has captured the attention of scientists and engineers for decades. We’re on our way, but not there yet; the technology is challenging. The fuel at the core of a fusion reactor on Earth must be heated to a scorching temperature of 180 million degrees Fahrenheit—much hotter than the sun. Then, once fusion begins, a lot of the energy needs to be contained in the system in order to sustain the reactions. Otherwise, the fuel cools off quickly when you stop putting in energy.

Among the laser-based research paths toward self-sustaining fusion, the most developed is the “central hot spot” (CHS) technique. In CHS, scientists use powerful laser beams to implode a ball of fuel, compressing it and igniting fusion at the same time. In fast ignition (FI), a related but more efficient approach, the target is first compressed by lasers and then fusion is ignited by a short pulse from a high-intensity laser.

For a variety of reasons, fast ignition hasn’t been pursued as aggressively as CHS in recent years, but that doesn’t mean it’s not a path to success. “If we want to produce clean, sustainable, and storable energy with fusion, alternative schemes such as FI are not only preferable but necessary,” says Ceurvorst. This new research, in which Ceurvorst participated while at the University of Oxford, explored some of the optimal parameters for FI. The team included researchers from the University of Oxford, Osaka University, University of California at San Diego, General Atomics, and University of Rochester.

One of the main challenges to FI occurs in the second stage of the process. When the fuel is compressed in the first stage, it becomes surrounded by plasma. Because plasma is electrically conductive, it can block or even reflect the laser light—so before sending your laser pulse in, you need to clear a path. This is called channeling, and is done by essentially drilling a hole through the plasma with a different kind of laser. It sounds simple enough in theory, but experimentally it’s not so straightforward.

In order to better understand the channeling process, the team performed experiments at the OMEGA EP laser facility* at the University of Rochester. OMEGA EP is an experimental tool that consists of several lasers and measurement devices, allowing researchers to bombard a target with different kinds of pulses from various lasers, and then analyze the impacts of each.

In this research, the team performed several trials that consisted of shooting a channeling laser pulse into hot plasma, followed by a probe pulse. The researchers systematically adjusted either the energy or the length of the channeling pulse in each trial, and then used the probe pulse to observe the newly formed channel's properties. From the results they determined three key things:

1. The ideal timing of the channeling pulse.
2. Where the channeling pulse should be focused.
3. The ideal intensity of the channeling pulse.

“This particular effort shows that we are able to maintain control of the channeling process, which means that we are able to produce high-quality channels that will keep our [beams] pointing in the correct direction,” says Ceurvorst. Moving forward, this information will take the guesswork out of channeling designs in FI experiments—enabling researchers to turn their attention to other challenges.

Working toward clean energy isn’t just a day job for Ceurvorst: it’s personal. He came to fusion research via a nontraditional path—Wall Street. After earning physics and math degrees and spending a few years in finance, Ceurvorst realized that he needed more personal stimulation and fulfillment. He applied to physics graduate school at the University of Oxford with the aim of studying fusion energy. After playing some catch up the first year, he says, everything clicked into place. “[S]ince then, I’ve been thrilled to work in a field which I am passionate about and which I feel will have profound positive impacts upon the world,” he said.

*To learn more about the laser lab that hosts OMEGA EP and its research value, and to sign a petition against its unfortunate loss of federal funding, visit the Laboratory for Laser Energetics website.